Jason J. Gurdak, Ph.D., U.S. Geological Survey
Hydrologic responses in the unsaturated zone and ground water to interannual, interdecadal, and multidecadal climate variability have important implications for resource sustainability, yet are poorly documented and not well understood in most aquifers. Natural climatic variability can augment or diminish ground-water pumping, and dramatic changes can occur in heavily pumped aquifers when climate cycles of different periods occur at the same time. With nearly one-third of all ground-water pumping in the United States occurring in the High Plains aquifer (450,700 km2) understanding subsurface hydrologic responses to climate variability is needed across this important ground-water resource. This presentation documents the role of interannual to multidecadal climate variability on ground-water level fluctuations, deep infiltration (3 to 23 m) events, and downward displacement of chloride and nitrate reservoirs in thick (15 to 50 m) unsaturated zones of the High Plains aquifer. Such responses are unexpected across much of the aquifer given a priori unsaturated total-potential profiles indicating upward water movement from the water table toward the root zone. Using singular spectrum analysis, variations were identified in all ground water and precipitation time series as partially coincident with known climate cycles, such as El Niño/Southern Oscillation. Climate-varying recharge and ground-water level fluctuations were most significantly correlated to the Pacific Decadal Oscillation. Using these findings, a new Hydrologic Time Series method is presented to estimate climate-varying recharge. The results indicate the importance of interannual to interdecadal climate variability on unsaturated water flux, and provided insight on process-level controls for the unexpected infiltration and chemical mobilization events. Estimated water fluxes (196 to 476 mm yr-1) are considerably larger than previous estimates of diffuse recharge, indicating the importance of preferential-flow on climate-varying recharge to the High Plains aquifer. Implications for sustainability of this aquifer are discussed in terms of future climate variability and change.
The 2007 Ground Water Summit
